The present invention relates to a gas turbine engine and is particularly concerned with a gas turbine engine that includes a cooling air flow control device which is particularly associated with the regulation of a cooling air flow for turbine components.
During a flight cycle of an aircraft a gas turbine engine is required to provide varied performance (e.g. takeoff and cruise) and its turbine stage, subject to a jet of hot gases, undergoes a range of operating temperatures. To control the component temperature, air is bled from a compressor stage to the turbine stage to maintain an even turbine temperature. Ducting air from the compressor reduces engine inefficiency and it is therefore desirable to minimize this bleeding whenever and wherever possible.
Conventional regulation means for cooling air flow is disclosed in GB2015085A wherein a cooling air control valve, remote from a component to be cooled, is closed at engine cruise conditions when an equilibrium of pressure exists either side of a diaphragm. An infra-red pyrometer detects turbine temperature change for engine conditions above cruise and sends a signal to an electro-magnetically operated pressure control valve releasing pressure from one side of the diaphragm thereby opening the cooling air control valve, increasing the cooling air flow above cruise air flow to the turbine stage.
The use of shaped memory metal (hereinafter termed SMM) components for controlling the fluid flow in a carburettor nozzle of an internal combustion engine is disclosed in GB2022710. A sleeve of SMM responds by changing the cross-sectional flow area according to the temperature of the fluid. The sleeve is relatively long in order to respond to the temperature of the fluid. In another embodiment of the invention, disclosed in GB2022710, a spring of SMM, located in the flow of fluid through the carburettor, controls the position of a needle relative to a seat thereby regulating the fluid flow.
It is important that turbine components, such as turbine blades, are cooled sufficiently for them to operate in extremely hot conditions and sometimes high centrifugal fields. Thermal cycling reduces the life of components so a constant temperature is beneficial. Allied to this, a constant component temperature is preferable in order to control thermal expansions and thus component to component clearances. The ability of the cooling system to reduce unnecessary compressor bleed and to minimise the response time of the system enhances engine efficiency and longevity of the engine components.
It is an object of the present invention to provide a cooling air flow control system for a gas turbine engine which maintains a constant temperature of an engine component, is rapid to respond throughout the operating conditions of the engine and promotes fuel efficiency of the engine.
According to the present invention there is provided a cooling air flow control device for a gas turbine engine, the cooling air flow control device comprising a component, a cooling passage defined within the component, a shaped memory metal valve, the shaped memory metal valve disposed in the cooling passage to regulate, in use, the flow rate of a cooling air flow supplied, in operation, through the cooling passage wherein the shaped memory metal valve operates by changing shape to control the flow rate of the cooling air flow in response to the temperature of the component.
Preferably the shaped memory metal valve comprises a body portion defining an orifice, the cooling air flow, in use, passing through the orifice.
Preferably the shaped memory metal valve comprises a shaped memory metal which is of washer-like shape.
Alternatively the shaped memory metal valve comprises a plate of shaped memory metal.
Alternatively the shaped memory metal valve comprises a plate of shaped memory metal, the plate is of generally arcuate shape.
Preferably the cooling air flow control device comprises the shaped memory metal valve arranged to operate by distorting substantially out of a principal plane of the shaped memory metal valve.
Alternatively the cooling air flow control device comprises the shaped memory metal valve arranged to operate by transforming shape substantially in a principal plane of the shaped memory metal valve when subjected to a predetermined temperature change.
Preferably the cooling air flow control device comprises the shaped memory metal valve arranged to operate by distorting substantially out of a principal plane of the shaped memory metal valve in the direction of the cooling air flow.
Alternatively the cooling air flow control device comprises a plurality of shaped memory metal valves which are disposed in the cooling passage.
Preferably the cooling air flow control device comprises a land, the land disposed in juxtaposition to the shaped memory metal valve.
Preferably the land is disposed upstream, relative to the cooling air flow, in juxtaposition to the shaped memory metal valve.
Preferably the component comprises a turbine blade.
Alternatively the component comprises a turbine vane, a turbine disc, an engine casing, a bearing chamber or an air heat exchanger.
Preferably the shaped memory metal valve changes, in operation, from a first shape to a second shape at a specific temperature.
Alternatively the shaped memory metal valve changes, in operation, progressively from a first shape to a second shape throughout a predetermined temperature range.